Drop Down list keep telling me there is a duplicate when there is not

Question

The goal was to change the items of a secondary drop down list when the item of the primary drop down list has changed.

With extremely confident and optimistic ideas, I came up with this simple code :

Model :

public class ChoixModel
{
    public INSC_Inscription inscription { get; set; }
    public List<Ecole> ecoles { get; set; }
    public List<Programme> programmes { get; set; }
    public int? ecoleChoi1 { get; set; }
    public int? programmeChoi1 { get; set; }
}

Controler :

    public ActionResult Phase1(int idInscrip)
    {
        inscriptionPrimaireEntities db = new inscriptionPrimaireEntities();
        ChoixModel model = new ChoixModel();

        if (model.ecoleChoi1 == null)
            model.ecoleChoi1 = db.Ecole.FirstOrDefault()?.id;

        model.ecoles = db.Ecole.ToList();
        model.programmes = db.Programme.Where(t => t.FK_Ecole == model.ecoleChoi1).ToList();

        return View(model);
    }

    [HttpPost]
    public ActionResult Phase1(ChoixModel model, string submiter)
    {
        inscriptionPrimaireEntities db = new inscriptionPrimaireEntities();

        if (!string.IsNullOrWhiteSpace(submiter))
        {
            if (model.ecoleChoi1 != null)
                model.programmeChoi1 = db.Programme.FirstOrDefault(t => t.id == model.inscription.Programme1)?.id;
        }

        return View(model);
    }

View :

@using System.Web.UI.WebControls
@using inscriptionPrimaire.Models
@model ChoixModel
@{
    ViewBag.Title = "Phase1";
    Layout = "~/Views/Shared/_Layout.cshtml";

    SelectList selectEcole = new SelectList(Model.ecoles, "id", "NomAffiche");
    SelectList selectPrograme = new SelectList(Model.programmes, "id","NomAffiche");
}

<form method="post" action="@Url.Action("Phase1", "Choix")" id="ViewModelForm">

<br /><br />

<table>
    <tr>
        <td style="min-width: 152px;"> 

        @Html.DropDownListFor(t => t.ecoleChoi1, selectEcole, new { @class = "form-control", style = "min-width: 150px;", onchange = "this.form.submit();" })

        </td>
        <td style="min-width: 152px;">

        @Html.DropDownListFor(t => t.programmeChoi1, selectPrograme, new { @class = "form-control", style = "min-width: 150px;" })

        </td>
    </tr>
</table>

</form>

The error occurs when I click to change the selected item of the primary drop down list Ecole. I suspect it has something to do with how I use submit on this :

@Html.DropDownListFor(t => t.ecoleChoi1, selectEcole, new { @class = "form-control", style = "min-width: 150px;", onchange = "this.form.submit();" })

Intended behavior was that when I change the selected item of drop down Ecole, then it would fill the drop down Programme with the actual programs offered by the Ecole selected.

Instead I have an error, an error that debug can't even catch, it shows only in the browser :

an item with the same key has already been added

So I went to look in my db if there was ANY duplicate... There is not even a single line in both tables that contains ANYTHING identical to the previous one.

I've been asked to show full stack trace (it's in french) :

Erreur du serveur dans l'application '/'.
Un élément avec la même clé a déjà été ajouté.
Description : Une exception non gérée s'est produite au moment de l'exécution de la requête Web actuelle. Contrôlez la trace de la pile pour plus d'informations sur l'erreur et son origine dans le code. 

Détails de l'exception: System.ArgumentException: Un élément avec la même clé a déjà été ajouté.

Erreur source: 

Une exception non gérée s'est produite lors de l'exécution de la requête Web actuelle. Les informations relatives à l'origine et l'emplacement de l'exception peuvent être identifiées en utilisant la trace de la pile d'exception ci-dessous.

Trace de la pile: 


[ArgumentException: Un élément avec la même clé a déjà été ajouté.]
   System.ThrowHelper.ThrowArgumentException(ExceptionResource resource) +52
   System.Collections.Generic.Dictionary`2.Insert(TKey key, TValue value, Boolean add) +12534430
   System.Collections.Generic.CollectionExtensions.ToDictionaryFast(TValue[] array, Func`2 keySelector, IEqualityComparer`1 comparer) +116
   System.Web.Mvc.ModelBindingContext.get_PropertyMetadata() +136
   System.Web.Mvc.DefaultModelBinder.BindProperty(ControllerContext controllerContext, ModelBindingContext bindingContext, PropertyDescriptor propertyDescriptor) +176
   System.Web.Mvc.DefaultModelBinder.BindProperties(ControllerContext controllerContext, ModelBindingContext bindingContext) +101
   System.Web.Mvc.DefaultModelBinder.BindComplexElementalModel(ControllerContext controllerContext, ModelBindingContext bindingContext, Object model) +55
   System.Web.Mvc.DefaultModelBinder.BindComplexModel(ControllerContext controllerContext, ModelBindingContext bindingContext) +1197
   System.Web.Mvc.DefaultModelBinder.BindModel(ControllerContext controllerContext, ModelBindingContext bindingContext) +330
   System.Web.Mvc.DefaultModelBinder.GetPropertyValue(ControllerContext controllerContext, ModelBindingContext bindingContext, PropertyDescriptor propertyDescriptor, IModelBinder propertyBinder) +17
   System.Web.Mvc.DefaultModelBinder.BindProperty(ControllerContext controllerContext, ModelBindingContext bindingContext, PropertyDescriptor propertyDescriptor) +377
   System.Web.Mvc.DefaultModelBinder.BindProperties(ControllerContext controllerContext, ModelBindingContext bindingContext) +101
   System.Web.Mvc.DefaultModelBinder.BindComplexElementalModel(ControllerContext controllerContext, ModelBindingContext bindingContext, Object model) +55
   System.Web.Mvc.DefaultModelBinder.BindComplexModel(ControllerContext controllerContext, ModelBindingContext bindingContext) +1197
   System.Web.Mvc.DefaultModelBinder.BindModel(ControllerContext controllerContext, ModelBindingContext bindingContext) +330
   System.Web.Mvc.ControllerActionInvoker.GetParameterValue(ControllerContext controllerContext, ParameterDescriptor parameterDescriptor) +331
   System.Web.Mvc.ControllerActionInvoker.GetParameterValues(ControllerContext controllerContext, ActionDescriptor actionDescriptor) +105
   System.Web.Mvc.Async.<>c__DisplayClass21.<BeginInvokeAction>b__19(AsyncCallback asyncCallback, Object asyncState) +743
   System.Web.Mvc.Async.WrappedAsyncResult`1.CallBeginDelegate(AsyncCallback callback, Object callbackState) +14
   System.Web.Mvc.Async.WrappedAsyncResultBase`1.Begin(AsyncCallback callback, Object state, Int32 timeout) +128
   System.Web.Mvc.Async.AsyncControllerActionInvoker.BeginInvokeAction(ControllerContext controllerContext, String actionName, AsyncCallback callback, Object state) +343
   System.Web.Mvc.Controller.<BeginExecuteCore>b__1c(AsyncCallback asyncCallback, Object asyncState, ExecuteCoreState innerState) +25
   System.Web.Mvc.Async.WrappedAsyncVoid`1.CallBeginDelegate(AsyncCallback callback, Object callbackState) +30
   System.Web.Mvc.Async.WrappedAsyncResultBase`1.Begin(AsyncCallback callback, Object state, Int32 timeout) +128
   System.Web.Mvc.Controller.BeginExecuteCore(AsyncCallback callback, Object state) +465
   System.Web.Mvc.Controller.<BeginExecute>b__14(AsyncCallback asyncCallback, Object callbackState, Controller controller) +18
   System.Web.Mvc.Async.WrappedAsyncVoid`1.CallBeginDelegate(AsyncCallback callback, Object callbackState) +20
   System.Web.Mvc.Async.WrappedAsyncResultBase`1.Begin(AsyncCallback callback, Object state, Int32 timeout) +128
   System.Web.Mvc.Controller.BeginExecute(RequestContext requestContext, AsyncCallback callback, Object state) +374
   System.Web.Mvc.Controller.System.Web.Mvc.Async.IAsyncController.BeginExecute(RequestContext requestContext, AsyncCallback callback, Object state) +16
   System.Web.Mvc.MvcHandler.<BeginProcessRequest>b__4(AsyncCallback asyncCallback, Object asyncState, ProcessRequestState innerState) +52
   System.Web.Mvc.Async.WrappedAsyncVoid`1.CallBeginDelegate(AsyncCallback callback, Object callbackState) +30
   System.Web.Mvc.Async.WrappedAsyncResultBase`1.Begin(AsyncCallback callback, Object state, Int32 timeout) +128
   System.Web.Mvc.MvcHandler.BeginProcessRequest(HttpContextBase httpContext, AsyncCallback callback, Object state) +384
   System.Web.Mvc.MvcHandler.BeginProcessRequest(HttpContext httpContext, AsyncCallback callback, Object state) +48
   System.Web.Mvc.MvcHandler.System.Web.IHttpAsyncHandler.BeginProcessRequest(HttpContext context, AsyncCallback cb, Object extraData) +16
   System.Web.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +103
   System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +155

HTML OUTPUT :

Not sure using your syntax how you would fix it, but I had similar issue and before populating (or repopulating) the drop down you need to do a call to empty out the .Clear() method. Then add them. This will ensure it is empty when adding them back — Brad, Mar 23 '18 at 18:54
Have you debugged to see if the `HttpPost` version of `Phase1` runs before the error?Or it never gets there? — JuanR, Mar 23 '18 at 19:08
I figured. The model binder is breaking due to your complex model. The question now is, what is breaking it? Can you post the HTML output of the dropdowns? — JuanR, Mar 23 '18 at 19:17
As you can see, by the way, the exception stack does prove useful... :-) — JuanR, Mar 23 '18 at 19:18
could we see your controller and/or model code handling the post to phase1/choix? that is where the error is occuring, when you're trying to re-build said list — Napoli, Mar 23 '18 at 19:32
@Napoli Well... it's there, te question is long, but the controller is there. And anyway, the POST action is never reached, something happens before I can get there — Antoine Pelletier, Mar 23 '18 at 19:34

score 3 · Accepted Answer · answered Mar 23 '18 at 19:42

The problem is you are using complex db entity models that the model binder is unable to properly reconstruct from the form data.

What I would suggest is that you avoid round-tripping entities and accept only the values you need. You can accomplish this by removing the model from the HttpPost method and accepting the values directly:

 [HttpPost]
 public ActionResult Phase1(int ecoleChoi1 , int programmeChoi1, string submiter)
 { 
      //... 
      //You can then reconstruct the model if necessary and send back
      ChoixModel model = new ChoixModel();
      //...
      return View(model);
 }

On the Razor view:

 @Html.DropDownList("ecoleChoi1 ", selectEcole, new { @class = "form-control", style = "min-width: 150px;", onchange = "this.form.submit();" })

 @Html.DropDownList("programmeChoi1", selectPrograme, new { @class = "form-control", style = "min-width: 150px;" })

If you debug this, you will find that the model binder passes back the values from the dropdowns.

Alternatively, since the two lists are not really editable fields but selection ones, you can remove them from the model and pass them in the ViewBag instead. That way you can keep accepting your model back.

Holy god, figuring this out would have taken me the rest of my life, I can't thank you enough... — Antoine Pelletier, Mar 23 '18 at 19:47

Drop Down list keep telling me there is a duplicate when there is not

1 Answers1